Shelly Bay 'primed' for coastal flooding

With land movement factored in, a 100-year coastal flooding event at Shelly Bay will occur every year by 2040, so why is the development proceeding, asks Rewi Newnham.

Water splashing in the ocean

Comment: Imagine buying a new coastal property on the premise that the flood risk was one in 100 years, only to find that within a few years those floods had become an annual event?

With construction of the controversial 350-house, $500 million development at Shelly Bay in Wellington now under way—despite sea-level rise projected to threaten this stretch of coastline in the coming decades—we must ask why is this development going ahead? Apart from some mana whenua voices, this question has been largely ignored but it’s now more relevant than ever.

Sea levels are rising globally because of warmer seawater expanding, melting mountain glaciers, and shrinking polar ice sheets. As a result of these three processes, the seas around New Zealand have risen by about 20cm since 1900, largely in line with the global average.

Shelly Bay has been able to cope with this rise without too much damage or erosion, thanks to its rocky coastline and a slightly elevated coastal platform resulting, ironically, from a combination of former sea-level change and tectonics.

However, the rate of global sea-level rise has now accelerated to over 3mm a year, largely because the previously stable polar ice sheets are now contributing more. Sea-level rise projections, based on future greenhouse gas emissions, typically range from 30cm to 100cm by 2100. The low end of the range is ‘locked and loaded’ as even current greenhouse gas levels will deliver at least this.

But that’s not all. As with so many climate change impacts, what really matters at the coast is what happens during extreme events.

To evaluate future coastal flooding impacts, we need to take these global sea-level projections and add on other locally important processes that, in combination, give rise to extreme events. These include regularly occurring spring and king tides that can reach up to 40cm higher than normal tidal cycle levels in Wellington Harbour.

Then there are storm surges when high winds and low air pressure combine to create a bulge in the level of the sea that is driven on to the coast. A storm surge in April 1968 elevated local sea levels in places by up to 88cm. Storm waves on Wellington’s south coast reached 12m and dealt a fatal blow to the Wahine interislander ferry and 53 of its passengers.

Long-term weather patterns such as the El Niño Southern Oscillation can also change the level of the sea over many years or even decades.

Extreme coastal flooding events occur when these ‘add-on’ processes coincide to locally raise the underlying sea level enough to flood the adjacent low-lying land. When this happens, topography is key to the extent of flooding and damage. The Shelly Bay development will be particularly vulnerable, being situated on a low-lying flat coastal strip of land, backed by steep cliffs.

As the sea continues to rise, the frequency and impact of coastal flooding will increase, but by how much?

A 2015 report by the Parliamentary Commissioner for the Environment projected changes in extreme coastal flooding events in Wellington and several other cities. These projections added the historical frequency and magnitude of storm surges, extreme high tides and waves to projected future sea-level rise. The results are sobering. For a 30cm rise in sea level, which will be reached by 2100 if not sooner, an extreme 100-year coastal flooding event in the past would be expected to occur every year in Wellington Harbour.

The threats don’t end there. A warming atmosphere holds more moisture—about 7 percent for every 1C of warming. The consequences have been severe in northern New Zealand this summer. As the oceans and atmosphere warm, increases in the amount and intensity of rainfall raise the risk of flooding in some areas.

Wellington Harbour is particularly susceptible to this process because it is mostly an enclosed basin, surrounded by steeply rising hills and fed at its northern end by a major river system draining an even bigger catchment area. The prospect of heavy rainfall and land-sourced floodwaters raises the risks of coastal inundation still higher, adding to the perfect storm scenario.

How fast the sea will rise in different places around the country also depends on whether the land is rising or falling. In parts of northern Europe, for example, sea-level rise is still being outpaced by the so-called glacial rebound—the land rising back after tens of thousands of years of subsidence under the weight of a vast ice sheet, now removed.

Glacial rebound is not likely to factor much in New Zealand but there is a much bigger threat here: vertical land movement, uplift or subsidence, resulting from tectonic processes, that can occur slowly over time or rapidly in earthquakes.

We only have to consider the dramatic changes to the coastline that accompanied the November 2016 Kaikōura earthquake to see how important this tectonic vertical land movement can be in changing shorelines dramatically. The problem is how to factor it in to provide more realistic sea-level rise projections.

The NZ SeaRise research programme has made a recent breakthrough. It combined satellite radar data with land-based navigation satellite measurements to calculate uplift and subsidence for the entire national coastline. This data was then fed into the latest global sea-level projections to show how vertical land movement affects sea-level change around our coasts.

In some areas, where the land is rising, the news is good as the land movement factor acts to slow down the rate of sea-level rise. But in other places, including Wellington where the land is subsiding, the opposite occurs.

Anyone can freely access this new tool and see for themselves just what sea-level rise combined with vertical land movement means for any point of interest.

At Shelly Bay, the most realistic global sea-level projection scenario (without the vertical land movement factor) predicts that sea level will be 30cm higher than today by 2080. But if land movement is factored in, the +30cm level is reached 40 years earlier, by 2040. And remember, this average rate doesn’t tell us about the impact that really matters—the frequency of coastal flooding.

With land movement factored in, a 100-year coastal flooding event at Shelly Bay will be occurring every year by 2040. The message should be clear for anyone considering investing in property at Shelly Bay: caveat emptor.

Read the original article at Newsroom.

Rewi Newnham is a professor in Physical Geography at Te Herenga Waka—Victoria University of Wellington.